The present invention relates to centrifugal compressors and diffusers and, more particularly, to a centrifugal compressor and a vaned diffuser which have performance thereof of high efficiency and low noise.
In centrifugal compressors, conventionally a vaned diffuser has been often used when high efficiency is required. These are disclosed in Japanese Utility Model Examined Publication No. 63-45599 and the like, for example. In such centrifugal compressor having the vaned diffuser, flow of fluid is guided by guide vanes so that flow angle measured from a tangential direction increases, and the flow of fluid is turned into a radial direction. Thus, the flow is efficiently decelerated. In this manner, performance of the centrifugal compressor having the vaned diffuser generally increases in efficiency, as compared with a case of a vaneless diffuser. In the vaned diffuser, however, throats are generally formed or defined respectively in passages between the adjoining guide vanes. Accordingly, choke occurs at large flow rate, and deceleration from a diffuser inlet to the throat increases at low flow rate so that the flow stalls and surge occurs. Thus, the prior art has a problem that operating range of compressor having the vaned diffuser is less than that of compressor having the vaneless diffuser. Further, flow of the fluid at an impeller outlet of the centrifugal compressor is non-uniform or uneven in a peripheral direction, that is, in a vane pitch direction. Accordingly, periodically fluctuating flow enters into the diffuser. Frequency of the fluctuating flow is equal to blade passage frequency of the impeller, that is, (the number of blades of the impeller).times.(rotational frequency). Accordingly, since the fluctuating flow impinges against the guide vanes of the diffuser, compressor with the vaned diffuser has a problem that high noise which prevails by a blade passage frequency component is generated as compared with compressor with the vaneless diffuser.
Moreover, in order to solve the problem that the operating range of compressor having the vaned diffuser is narrow, a diffuser which is low in solidity (vane length/cascade average pitch) has been invented and disclosed in Japanese Patent Unexamined Publication No. 53-119411. In the vaned diffuser, throats are not formed in passages between blades. Accordingly, choke and surge are difficult to be generated. Thus, the operating range of compressor with low salidity vaned diffuser is wide in the same degree or extent as that of compressor with the vaneless diffuser and is high in performance, but a problem that noise is high has not been solved. Furthermore, as disclosed in Japanese Utility Model Unexamined Publication No. 63-9500, a diffuser has also been proposed in which long guide vanes and short guide vanes are alternately arranged, and radii of tailing edges of all of the guide vanes are made constant. Further, as disclosed in U.S. Pat. No. 4,824,325, a vaned diffuser has also been proposed in which guide vanes having low solidity are provided in two rows, a space in a radial direction is provided between a first row of guide vanes and a second row of guide vanes, and the number of guide vanes of the first row is twice the number of guide vanes of the second row. Centrifugal compressors having such diffusers are characterized to have wide operating range as compared with a centrifugal compressor having a conventional vaned diffuser. However, a problem that noise is high has not been solved.
As described above, the centrifugal compressor provided with the vaned diffuser is generally high in efficiency as compared with the centrifugal compressor which has the vaneless diffuser, but has a disadvantage that the operating range is narrow. A centrifugal compressor which has solved the problem to have a wide operational range has been proposed. However, a problem that the noise is high has not still been solved.